REGENERATION MEASURES AND THE FORMATION OF SALTPETRE CV 
(4) Where the covering of raw humus is somewhat more strongly 
developed, chequerboard cutting, shelterwood cutting or clear 
Fscutting does not by itself produce a formatron of saltpetre. he 
formation of ammonia, however, is substantially increased. But 
the formation of saltpetre can be produced either by a prepa- 
ration of the ground with machines or by the burning of brush- 
wood or by the denshiring of the ground. 
(sInipiotest. types, where the humus-nitrogen is) converted to 
nitric acid, for instance, in forests of the finer kinds of leaf-trees 
Oman mesbulent sprucen totests, the formation Of Saltpetretisktun. 
creasedrsimply by tNetinereased access of light whyehiis ar com 
sequence of chequerboard cutting or clear cutting. 
In what follows I shall seek to develop more fully the causes of these 
notable changes. 
CHaAP. XI. On the active factors in the changes in the trans- 
formation of nitrogen. 
The change in the conversion of nitrogen which has just been described 
presupposes such alterations in the ground that saltpetre-forming bactéria there 
can develop their activity. We may remind the reader at the outset that 
these bacteria are very widely distributed in nature. When we cannot find 
traces of their existence in certain kinds of soils, for instance, in the humus- 
covering of mossy coniferous forests, this shows that they do not there find 
suitable conditions of development. After the humus-covering has undergone 
certain changes favourable to the bacteria, they rapidly make their appear- 
ance, which in a measure forms some evidence as to how generally these 
bacteria are distributed in nature and how very easily they are spread. We 
need only call to mind the fact that nitrification makes its appearance very 
soon in a windfall in the forest, although nitrification-organisms may be com- 
pletely lacking within large areas of the surrounding woodlands. 
The matter is probably most correctly explained on the supposition that an 
infection of the ground is constantly going on, but that where the soil is 
of a nature unsuitable for nitrifying agents, they perish as soon as they ap- 
DE 
In order to make this phenomenon clear, it might be expedient to give 
some account of some investigations by the Danish soil bacteriologist, Ha- 
RALD C. CHRISTENSEN (1915, pages 4— 54) on the nitrogen-assimilating bac- 
terium”  azotobacter. This bacterium is widely distributed, but occurs only 
im soil that is rich in bases, especially in lime. It therefore never occurs in 
sour soil, seldom in neutral, but practically always in soil with alkaline re- 
action. His searching enquiries show that azofobacter really perish in a 
soil which is poor in lime. The bacterium rapidly undergoes pathological 
changes and dies, while it can be kept for years in a sample of soil that 
contains sufficient carbonate of lime. It is evident that under such circum- 
" Such infection might perhaps explain the fact that, when samples of raw humus are 
stored, it is possible to observe in them a very weak nitrification. 
